Examples of the silk used in experiments to detect damage in composites, shown under black light. (Left) Ordinary fibroin of the Bombyx mori silk worm. The observed fluorescence is the result of molecules already present in the protein structure of the fiber. (Middle) Mechanophore-labeled silk fiber fluoresces in response to damage or stress. (Right) Control sample without the mechanophore. Credit: Chelsea Davis and Jeremiah Woodcock/NIST

Consumers want high-performance sporting goods and fuel-efficient vehicles, municipalities desire weather-resistant bridges, and manufacturers need efficient ways to make reliable aircrafts and cars. The requirement is new energy-saving, lightweight composites that would resist crack or break even when exposed to structural or environmental stress for prolonged time periods.

The probe, called a mechanophore, could potentially shorten the time and reduce the materials required for developing many types of new composites, and speed up product testing.

The rhodamine spirolactam (RS) dye, which changes from a dark state to a bright state on the application of a force, was used by the NIST team to create the probe. In this experiment, the silk fibers present within the epoxy-based composite were used to bind the molecule.

Increase in the force applied on the composite resulted in activation of the RS by the strain and stress, allowing it to fluoresce when excited with a laser. Although the change was invisible to the naked eye, NIST built and designed a red laser and a microscope to take photographs inside the composite.

The images displayed the most minute fissures and breaks to its interior, and exposed the points in which the fiber had broken. The results were featured in the journal Advance Materials Interfaces.

Various types of materials were employed to design the composites. Natural composites such as elephant tusk (bone) or crab shell are made of polysaccharides and proteins. In this research, silk filaments prepared by Professor Fritz Vollrath’s group at the Oxford University using Bombyx mori silk worms were combined with epoxy.

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This type of fiber-reinforced polymer composite brings together the most beneficial features of the main components: the toughness of the polymer and the strength of the fiber. All composites have an interface in which the components meet. A composite’s ability to resist damage depends on the resilience of the interface. Designers and manufacturers often prefer thin, flexible interfaces, although it is not easy to measure the interfacial properties in a composite.

There have long been ways to measure the macroscopic properties of composites. But for decades the challenge has been to determine what was happening inside, at the interface.

Jeffrey Gilman, NIST

Optical imaging is an option to overcome this challenge. Traditional optical imaging methods can only record images in the range of 200 - 400 nanometers. Such methodologies are not effective for imaging the interphase in composites measuring 10 to 100 nanometers in thickness. However, the researchers could “see” damage exclusively at the interface by using optical microscopy, when the RS probe was installed at the interface.

The NIST research team is thinking of expanding their research to find out how such types of probes could be employed for other composites also. They also wish to employ such types of sensors to improve the capability of these composites to bear extreme heat and cold.

Additionally, there is also a great demand for composites that can resist prolonged exposure to water, particularly for use in building more resilient infrastructure components such as bridges and large-sized blades for wind turbines.

The researchers plan to continue their search for more ways that damage sensors such as the one in this research could enhance standards for currently-used composites and develop new standards for future composite materials, so that the materials are strong, safe, and reliable.

We now have a damage sensor to help optimize the composite for different applications. If you attempt a design change, you can figure out if the change you made improved the interface of a composite, or weakened it.

Jeffrey Gilman, NIST

Collaborative research agreements with both the Army Research Office and the Air Force Office of Scientific Research funded this research.

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